1. Field of the Invention
The present invention relates to a method of introducing into fresh concrete, as herein defined, compounds capable of sequestering chloride ions to establish resistance to corrosion of metal reinforcing elements contained within or contacting the concrete and provide a corrosion resistant oxide layer on the metal reinforcing elements, as well as related compositions and structures. The invention is also directed toward corrosion protection of concrete articles wherein the concrete has already set and hardened.
2. Description of the Prior Art
The advantageous use of metal reinforcing members, such as steel reinforcing members, in concrete for structural uses has been known for many years. Concrete is known to provide desired compressive strength, but tends to lack tensile strength. The reinforcing bars co-act with the concrete to provide enhanced tensile strength for the combination of materials. It has also been known to employ corrugated metal deck in combination with concrete to create a composite with similar benefits. Numerous other metal members have been embedded in concrete or provided in contact therewith to achieve enhanced benefits in the structural environment as a result of such materials. Among these additional materials are grids, beams, bolts, hold-downs and wire mesh.
One problem with such construction has arisen as a result of exposure of concrete to salts, such as calcium chloride and sodium chloride, on external structural members to resist the undesired accumulation of snow and ice on bridges and other concrete paved areas such as roadways, parking lots, sidewalks and the like. While these chloride salts do provide benefits in terms of de-icing of concrete surfaces, they frequently result in the chloride solutions migrating into the concrete decks and adjacent vertical concrete surfaces, such as walls and columns, also subjecting these to chloride intrusion. Also, saline seawater may migrate into the pores of concrete exposed to seawater as in sea walls, With respect to bridge decks, in particular, an enhanced problem results from air movement under the deck creating an environment wherein the salts are aspirated into the concrete and salt laden solutions flow into the pores.
Regardless of the manner in which chloride enters such concrete, the chloride, upon reaching the steel reinforcing members, tends to accelerate corrosion of the same because the oxidation of the metal metallic iron to Fe2+ is catalyzed by the chloride. Also, oxides and hydroxides of Fe2xe2x88x92 frequently form and occupy porosity in the vicinity of the interface of the steel and concrete. In addition, oxides and hydroxides of Fe3+ may also be produced. As these iron oxides and hydroxides are of greater volume than the iron metal from which they were produced, they tend to cause internal stresses which may become high enough to crack the concrete, and also degrade the desired bond between the metal reinforcing elements and the concrete.
U.S. Pat. No. 5,049,412 discloses a method of re-alkalizing concrete in which carbonation has occurred. An outer layer of the concrete structure containing reinforcement which layer through exposure to air has been carbonated has an adjacent layer that remains relatively less carbonated. The patent discloses applying to the outer surface a water type adherent coating followed by introducing between the outer adjacent layers, water from a source external to the concrete structure and maintaining the concrete structure in this condition for a period of time sufficient to effect diffusion of the alkaline materials from the relatively less carbonated adjacent layer into the relatively carbonated outer layer.
U.S. Pat. No. 5,198,082 discloses a process for rehabilitation of internally reinforced concrete, which includes temporary application of an adhered coating of an electrode material to surface areas of the concrete. Distributed electrodes such as a wire grid is embedded in the coating. A voltage as applied to the reinforcement and distributed to the electrode to cause migration of chloride ions from the chloride into the electrolytic coding. Among the shortcomings of this approach are the need to provide, at the local source, a source of electrical power. This electrical equipment might have to be maintained at the site for extended periods of time. This further complicates matters by establishing a risk of injury to children and others that might find the equipment at an attractive nuisance, as well as the risk of theft and vandalism. Also, such chloride extraction processes may alter the concrete microstructure by making it more porous and permeable, thereby, facilitating enhanced re-entry of chloride when de-icing salts are again applied to the exterior.
It has been known to employ nitrites, such as calcium nitrite, in resisting corrosion of steel parts in concrete. It is believed that the nitrites oxidize the Fe2+ to Fe3+ which, in turn, precipitates as Fe2O3. The Fe2O3 thus formed tends to act as a barrier to further contact between the chloride and the steel. See, generally, U.S. Pat. Nos. 4,092,109 and 4,285,733. Neither calcium nitrate nor Fe2O3, however, function to sequester chloride. The latter provides merely a barrier.
There remains, therefore, a very real and substantial need for a method and related composition and structure which will resist undesired corrosion of metal structural elements contained within, or in contact with, concrete structural members.
The present invention has met the above-described need.
The method, in one embodiment, includes resisting corrosion in concrete containing metal reinforcing elements composed of steel, copper, galvanized steel, tin plated steel or other structurally suitable metals by introducing into fresh concrete containing metal reinforcing elements at least one compound capable of sequestering chloride ions in a low solubility compound.
In connection with steel reinforcing elements, a low solubility compound within which the chloride ions are sequestered preferably also is created in a reaction that releases nitrite, which serves to oxidize Fe2+ to thereby provide a corrosion-resisting oxide layer on the steel reinforcing elements. This, therefore, in connection with steel achieves two levels of corrosion resistance, one of which is the actual capturing or sequestering of the potentially damaging chloride ions, and the second of which provides a protective layer on the metal reinforcing elements.
Among the preferred compounds for use in the method of the present invention are one or more compounds selected from the group consisting of 3CaOxe2x80xa2Al2O3xe2x80xa2Ca(NO2)2xe2x80xa2nH2O; 3CaOxe2x80xa2Al2O3xe2x80xa2Ca(NO3)2xe2x80xa2nH2O; and 3CaOxe2x80xa2Fe2O3xe2x80xa2Ca(NO2)2xe2x80xa2nH2O; wherein n=0 to 18 and preferably 10 to 18, depending upon the relative humidity to which a compound is equilibrated. If desired, lower values of xe2x80x9cnxe2x80x9d may be obtained by drying at low relative humidity as by evacuation or by heating, for example.
A further compound employed in another embodiment of the invention is, 3Me(II)Oxe2x80xa2R2O3xe2x80xa2Me(II)(anion)2xe2x80xa2nH2O wherein Me(II) is one or more divalent cations, such as Ca for example, R2 is Al2, Fe2 or Cr2 anion is NO2, NO3, CO3, BO4 or OH and n is 0 to 24, and preferably 10 to 24. For some formulations, the anion may be divalent. In this case the formula would be Me(II)Oxe2x80xa2R2O3Me(II)(anion)nH2O wherein n is 0 to 18 and prefer 10 to 18.
The invention also contemplates a concrete structure which has hydrated fresh concrete and a plurality of metal structural elements in contact with the hydrated fresh concrete with a compound which sequesters chloride ions dispersed within the concrete.
The invention in another embodiment contemplates rehabilitation of existing concrete structures by providing a chloride sequestering compound in a member adjacent to the concrete structure and having a composition such that migration of chlorine ions away from metal structural elements in the concrete structure and into the adjacent overlay may be effected. In addition, if desired, release of nitrite to migrate into the concrete structure and afford corrosion protection to embedded steel.
In one version, the overlay, which may be formed in situ or as a preformed panel, contains the chloride-sequestering compound. In another, a slurry may be applied to the concrete structure with or without an overlay secured thereover.
It is an object of the present invention to provide a method and related compounds and structures for inhibiting corrosion of metal elements positioned within or in contact with concrete in a structural environment.
It is a further object of the present invention to provide such a system wherein undesired chloride ions will, as a result of a reaction, be sequestered, thereby reducing their ability to corrode the metal elements.
It is yet another object of the invention to, through a reaction effecting such sequestration of ions, to provide free nitrites which will oxidize the Fe2+ to a Fe3+ ion which, in turn, precipitates as Fe2O3 which coats the metal element and, thereby, resists corrosion.
It is yet another object of the present invention to provide such a system which employs unique compounds.
It is another object of the present invention to provide such a system which will effectively and rapidly provide corrosion resistance to steel and other metals.
It is yet another object of the invention to provide such a system which may be employed by merely adding one or more compounds of choice to fresh concrete without requiring substantial changes in conventional practices employed in producing and placing the concrete structure.
It is a further object of the present invention to provide such a system where an existing concrete structure may be rehabilitated through by sequesting the chloride and providing a means to accumulate nitrite ions in the vicinity of the embedded steel. It is appreciated that the nitrite ions oxidize presently corroding steel to produce a protective layer. In some formulations nitrite ions may not be available and in these instances rehabilitation is the result of chloride sequestration only.
It is yet another object of the present invention to provide such a system wherein an overlay, which contains a composition which may be of the type employed in other embodiments of the invention and facilitates sequestering of chloride and corrosion protection of metal structural elements. In another version, a slurry containing the compound of interest may be applied to the concrete structure with an overlay material either formed in situ or as a preformed panel secured thereover.
It is yet another object of the present invention to provide such a system for rehabilitation of existing concrete structures without requiring a source of electrical energy to be present on an ongoing basis during the performance of the method.
These and other objects of the invention will be more fully understood from the following description of the invention with reference to the drawings appended hereto.